1. Field of the Invention
The present invention relates to a membrane/electrode assembly for a polymer electrolyte fuel cell.
2. Discussion of Background
In recent years, a polymer electrolyte fuel cell is required to be operated in higher temperature and lower humidity environment especially for automobiles. Accordingly, as a proton conductive polymer contained in a polymer electrolyte membrane and a catalyst layer of a membrane/electrode assembly, a material exhibiting high electrical conductivity in a low humidity environment has been desired.
To improve electrical conductivity of a proton conductive polymer, the number of ionic groups should be increased. However, an increase of ionic groups increases the water content of the proton conductive polymer. If the water content of the proton conductive polymer to be used for a catalyst layer is too high, the proton conductive polymer will swell, whereby air gaps in the catalyst layer may be clogged. As a result, the diffusability of a gas to be supplied will decrease, whereby a so-called flooding phenomenon such that the power generation performance of a polymer fuel cell remarkably decreases will occur.
As a proton conductive polymer to be used for a catalyst layer, the following polymers have been proposed.
(1) A copolymer of tetrafluoroethylene with CF2═CF— (OCF2CFX1)m—Oq—(CF2)n-A (wherein X1 is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, n is an integer of from 1 to 12, q is 0 or 1, and A is a sulfonic acid type functional group) (Patent Document 1).
(2) A copolymer represented by —[CF2—CF2]1.00—[CF2—CF (—O(CF2)2—SO3H)]2.19— (Patent Document 2).
The polymer (1) is a polymer having a very high water content so as to maintain high electrical conductivity in a low humidity environment, whereby flooding is likely to occur.
The polymer (2) has insufficient electrical conductivity, and if the ion exchange capacity of the polymer is increased so as to increase the electrical conductivity, it will have a too high water content, whereby flooding is likely to occur.
Patent Document 1: JP-A-6-260185
Patent Document 2: JP-A-2004-273257